Additive layer manufacturing methods can be broadly divided into two groups. In a first group, material is deposited sequentially in patterned layers, the pattern of each layer representing a two dimensional cross section of a three dimensional shape of an object. As each layer is deposited atop a previous layer, a three dimensional object is built. Examples of this group of methods include; direct energy deposition (where focussed thermal energy is used to fuse materials as they are being deposited), material extrusion (where an extrusion head moves in a pattern selectively dispensing material through an orifice as it travels) and sheet lamination (where sheets of material already defining a two-dimensional pattern are bonded in sequence to build up the three dimensional object.)
In the second group, the process starts with a bulk mass which may, for example, be a bed of powdered material such as a ceramic, a ferrous alloy or a non-ferrous alloy, or a vat of liquid typically comprising a photopolymer. Regions within the mass are selectively treated, for example by melting, sintering, photochemical reaction or interaction with a chemical bonding agent, to solidify. However unlike with the first group, the untreated material remains in a layer as the next layer is formed. Surplus (untreated) material may be removed when the three dimensional build is complete, through it also known for surplus material to sometimes be contained within cavities of the three-dimensional object.
Within the second group of ALM methods, a layer of untreated material may be deposited onto an already treated layer by dispersal from a hopper or by dipping the treated layer below the surface of the untreated material. For the three-dimensional shape to be properly controlled, the thickness and consistency of thickness of each layer to be treated must be carefully controlled. This is conventionally achieved by positioning a top surface a fixed distance from the tip of a “doctor” across a top surface of the material to level out the top surface.
Various forms of doctor are known. In its simplest form the doctor is a straight-edged, rigid blade which is skimmed across the material surface gradually pushing away any excess from the newly added material layer. In a more advanced form, the straight edged, rigid blade may comprise a hollow through which a vacuum can be applied. The blade is positioned a small distance from the required top surface level. Rather than push material away, such a device sucks excess material into the hollow as it passes across the surface. This can result in dips in the surface. The vacuum blade is subsequently lowered to the required surface level and passes over the surface again. In regions where there is insufficient material remaining at the top surface, material is drawn from the hollow back onto the surface. Where the material to be treated is viscous, there may be a waiting time between recoating and treating while the viscous top surface drains and settles. In some cases highly viscous materials may not drain and settle resulting in the top surface rising above the desired height and a consequent, detrimental effect on the quality of the component.
One challenge with known doctor devices is stresses seen in an already treated layer when the doctor device passes over a successive layer. High stresses can result in damage to the component. Factors known to influence such stresses include: blade speed, material type, the gap between the blade and the treated surface, and the structure of material around and within the blade.